A $12 ext{ V}$, $60 ext{ W}$ lamp is connected to the secondary of a step-down transformer, whose primary is

Question

A $12	ext{ V}$, $60	ext{ W}$ lamp is connected to the secondary of a step-down transformer, whose primary is connected to ac mains of $220	ext{ V}$. Assuming the transformer to be ideal, what is the current in the primary winding?

Accepted Answer

For an ideal transformer, input power equals output power. $P_{in} = P_{out} = 60	ext{ W}$. Since $P_{in} = V_p 	imes I_p$, we have $60	ext{ W} = 220	ext{ V} 	imes I_p$. Thus, $I_p = \frac{60}{220} \approx 0.27	ext{ A}$.

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